A source of wonder for nearly 5,000 years, the megaliths arranged in a circle that characterize Stonehenge are attributed by Arthurian legend to the magician Merlin who, in the Middle Ages, is said to have stolen the monument in Ireland.
Even today, the ensemble fascinates the general public and scientists alike, particularly with regard to the provenance of the stones. Recently, scientists determined that the iconic vertical sandstones of the Neolithic site came from the Marlborough region of England, while the bluestones near the center came from Wales. But the origin of the altar stone, a single six-ton slab lying on its side at the very heart of the circle, remains a mystery.
A “completely sensational” discovery
It was long thought that it also came from Wales, but tests carried out in this direction never “gave anything”, underlines Richard Bevins, professor of geology at the University of Aberystwyth and co-author of the study published in the journal Nature.
These British and Australian researchers broadened their horizons and made a “quite sensational” discovery, he said. Through chemical analysis, they determined that the altar stone came from the Orkney Basin in northeastern Scotland, at least… 750 kilometers from Stonehenge!
Scientists were stunned by the discovery, as the distance was the furthest a stone had ever travelled at the time, said Nick Pearce of Aberystwyth University, a co-author of the study.
A mysterious journey
Whether humans 4,500 years ago were capable of transporting such enormous stones from Wales has previously been the subject of heated debate among archaeologists and historians.
The fact that a five-metre by one-metre block could span most of the United Kingdom indicates that society in the British Isles during the Neolithic period was highly organised and connected, the researchers say.
One theory is that the stone was brought to southern England not by humans, but by natural ice flows. However, research has shown that ice would have transported such stones “northwards, away from Stonehenge,” geologist and lead author of the study Anthony Clarke of Australia's Curtin University said at a news conference.
Overland transport seems extremely difficult for the time, as dense forests, marshy peat bogs and mountains posed “formidable barriers” to prehistoric movers, Clarke also explains.
There remains the possibility that the stone was transported by sea. There is indeed evidence of the existence of a “vast Neolithic maritime transport network” which allowed the transport of pottery and precious stones throughout the region, according to the geologist.
Laser beam tests
To determine the mineral's origin, the researchers shot laser beams into the crystals of two thin slices of the altar stone. The ratio of uranium to lead in these crystals acts like a “miniature clock” for the rocks, helping to determine the age of the stone, said Chris Kirkland of Curtin University, a co-author of the study.
The team then compared this age to other rocks in the UK and concluded “with a high degree of certainty” that the stone came from the Orkney Basin. The scientists want further research to find out which Scottish quarry the slab came from and how it got to Stonehenge.
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